The present invention relates generally to an optical delay line apparatus and, more particularly, to a recirculating binary fiberoptic delay line apparatus for time steering of array antennas.
The state of the art of optical delay line apparatus is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Pat. Nos.:
U.S. Pat. No. 3,368,202 issued to Crousel on Feb. 6, 1968;
U.S Pat. No. 4,671,604 issued to Soref on Jun. 9, 1987;
U.S. Pat. No. 4,671,605 issued to Soref on Jun. 9, 1987;
U.S. Pat. No. 4,714,314 issued to Yang et al on Dec. 22, 1987;
U.S. Pat. No. 4,725,844 issued to Goodwin et al on Feb. 16, 1988;
U.S. Pat. No. 4,814,773 issued to Wechsberg et al on Mar. 21, 1989;
U.S. Pat. No. 4,814,774 issued to Herczfeld on Mar. 21, 1989; and
U.S. Pat. No. 4,832,433 issued to de La Chapelle et al on May 23, 1989.
The Crousel patent is directed to a memory core matrix and simple delay means like magnetic drum apparatus or sonic delay lines in place of the complicated interconnection network, summation equipment and shift registers which are employed in a multibeam receiving system.
The Soref patent (604) describes a wavelength dependent, tunable, optical time delay system for electrical signals having a conversion/tuning unit for converting an incoming electrical signal into an optical signal as well as selectively varying the wavelength of the optical signal. By selectively varying the wavelength of the optical signal, the electrical signal can effectively and rapidly time delayed as desired in response to the electronic signal.
The Soref patent (605) is directed to a length dependent, optical time delay/filter device for electrical signals made up of a plurality of optical fibers of varying lengths. Depending upon which fibers an optical signal (converted from an incoming electrical signal) passes through determines the time of travel of the optical signal through the device, and as a result thereof is time delayed.
The Yang et al patent discloses a mode dependent, optical time delay system for electrical signals having a highly multi-mode optical fiber having a step index profile in optical alignment with an optical source which is capable of converting an incoming electrical signal into an optical signal.
The Goodwin et al patent discusses a technique for applying selected phase delays to an optical carrier signal, the phase delays being referenced to a radio-frequency (rf) subcarrier signal. The optical signal to be phase delayed is introduced into a phase delay network comprising multiple optical paths and multiple electro-optical switches, controllable by signals generated in switching logic. The selected delays can be introduced for purposes of data modulation, or for steering an antenna beam in a phased-array antenna.
The Wechsberg et al patent describes a fiber optic feed network for a radar which couples the antenna with the transmitting and receiving circuitry. The feed system includes a set of optical multiplexers interconnected by sets of optical fibers. Microwave energy of the radar is converted to optical radiation for communication to the antenna, and then converted back to the microwave energy.
The Herczfeld patent discloses an optically controlled phased array antenna system and method of operating same utilizing fiber optic transmission lengths and controlled piezo-electric crystals or equivalent elements to introduce predetermined time delays into each light signal by controlling the respective length of each fiber optic link. The light carrying fibers are wrapped around the respective crystals in accordance with a pattern to introduce time delays corresponding to the amount of stretch given to the fiber by the energized crystals. Beam scanning is achieved by controlling the matrix of crystals to introduce appropriate time delays into the optical signals which drive the respective antenna elements.
The de La Chapelle et al patent is directed to a fiber-optic feed network using series/parallel connected light emitting optic-electronic components, such as laser diodes for distributing RF, microwave, MMW, digital signals, and pulse modulated light. The diodes are selected in number and impedance to provide a good wideband impedance match to the RF/microwave/MMW/digital driving source.
Much effort has been devoted for the development of efficient fiberoptic delay line architectures that can provide a programmable delay over a wide range of delays. The most efficient such architecture is the binary programmable fiberoptic delay line (BIFODEL). In the binary programmable fiberoptic delay line the microwave signal to be delayed, linearly modulates the intensity of a laser diode. The resultant optical signal, is then routed through M fiber segments. Each fiber segment has a length that is equal to twice the length of its right neighbor. The length of the smallest or rightmost fiber segment is such that the delay which it introduces corresponds to the desired delay resolution T.sub.o. Selection of the fiber segments, through which the signal is routed, can be achieved by means of M 2.times.2 optical switches. Each switch allows the signal to enter or to bypass a specific fiber segment. Thus, by selecting the states of the switches, a delay T may take any value (in increments of T.sub.o) that is equal to or less than the maximum value which is equal to EQU T.sub.max =(2.sup.M -l)T.sub.o ( 1.0)
After the signal has been routed through the proper fiber segments, it is detected and subsequently buffered and further processed. It may be noted that the importance of the binary programmable fiberoptic delay line comes because the total number of delays that can be generated is equal to 2.sup.M, and thus, with a small number of fiber segments, a very large number of delays can be generated, e.g., for M=10 we have 1024 possible delays. Note we have demonstrated (at the RaD Center) a prototype binary programmable fiberoptic delay line that operates with M=8 and over the 500-1000 MHz band.
In a one dimensional antenna array with K elements, the maximum time delay required by the eth element is given by EQU T.sub.imax =d.sub.i sin.theta..sub.M /c, (1.1)
where d.sub.i is the distance of the ith element from the Oth or reference element, c is the speed of light and .theta..sub.M is the maximum angle (with respect to boresight that the antenna might be steered to. For an isotropic one dimensional array with element-to-element distance of d=.lambda./2, where .lambda. is the wavelength of the RF radiation, Equation 1.1 becomes EQU T.sub.imax =i.lambda.sin.theta..sub.M /2c, (1.2)
Equation 1.2 indicates that the maximum delay for .theta..sub.M is different for each of the K elements. In a similar manner, it can be shown that the delay resolution is different for each element, and is proportional to EQU T.sub.imin =i.lambda.sin.theta..sub.o /2c, (1.3)
where .theta..sub.o is the steering angle resolution. It is worthwhile to examine an example in order to understand the rather severe delay line requirements for a one dimensional antenna. Let us assume that K=101, .lambda.=0.3 m (i.e., L-band or f=1 GHz), .theta..sub.M =45*, and .theta..sub.R =0.176*. For this scenario and for each element we require a binary programmable fiberoptic delay line with M cascaded delay stages, where M is equal to EQU M=log.sub.2 (45/0.176)=8 (1.4)
It is important to note that the delay and delay resolution characteristics of the binary programmable fiberoptic delay line are different for each element. For example, for the first element we require a maximum delay of 0.35 nsec with a resolution of 1.54 psec, for the 10th element we require a maximum delay of 3.5 nsec and a resolution 15.4 psec, and for the 100th element we require 35 nsec maximum delay with 154 psec resolution.
The above discussion clearly indicates that a different binary programmable fiberoptic delay line is required for each array element. Note that a similar situation exists with 2-D antennas as well, with the hardware complexity requirement being proportional to K.sup.2, where KxK are the array dimensions. This situation is obviously impractical because it may result in a large amount of hardware. This is because current and future 1- and 2-D antenna arrays with several thousand elements are rather typical which implies that several thousand different binary programmable fiberoptic delay lines would be required.
To the best of our knowledge, there has been no delay line architecture that addresses this problem. In this disclosure we present a wavelength-coded cascaded BIFODEL delay line architecture (WC-BIFODEL) that is capable of addressing K different elements while requiring log.sub.2 K, rather than K, different binary fiberoptic delay lines.
While the above-cited references and discussions are instructive, there still remains a need to provide a recirculating binary fiberoptic delay line apparatus for time steering of very large array antennas. The present invention is intended to satisfy that need.